1. Field of the Invention
The invention relates to a method and to an arrangement for scalable confocal interferometry for distance measurement, for 3-D detection of an object, for OC tomography with an object imaging interferometer and at least one light source.
2. Description of the Related Art
The sequential recording of data from different depths of the object space by focusing through plays a functionally important role, as is known, for microscopic white-light interferometry. Reference to this can be found in the following documents:    [1] Balasubramanian N: Optical system for surface topography measurement. U.S. Pat. No. 4,340,306 (1982),    [2] Kino G S, Chim S: Mirau correlation microscope. Appl. Opt. 29 (1990) 3775-3783,    [3] Byron S L, Timothy C S: Profilometry with a coherence scanning microscope. Appl. Opt. 29 (1990) 3784-3788,    [4] Dresel T h, Häusler G, Venzke H: Three-dimensional sensing of rough surfaces by coherence radar. Appl. Opt. 31 (1992) 919-925,    [5] Deck L, de Groot P: High-speed noncontact profiler based on scanning white-light interferometry. Appl. Opt. 33 (1994) 7334-7338,    [6] Windecker R, Haible P, Tiziani H J: Fast coherence scanning interferometry for measuring smooth, rough and spherical surfaces. J. Opt. Soc. Am 42 (1995) 2059-2069.
Approaches [2], [3], [5], [6] relating to white-light interferometry, which is often also referred to as short-coherence interferometry, are in general in fact restricted to the microscopic range. These approaches cannot be scaled to a major extent in terms of resolution capability and depth measurement range in the direction of coarser scales since these methods are in general very closely linked to the magnitude of the light wavelength that is used. Short-coherence interferometry in the infrared spectral range generally leads to a multiplicity of technical problems and to high costs.
In addition, the approaches [1], [2], [3], [4], [5] and [6] as well as the approach by G. Häusler, described in DE 10 2005 023 212 B4 [7], can be miniaturized in a measurement arrangement or sensor arrangement only to a limited extent since, in this case, the object arm or the reference arm of the interferometer must be constructed with moving components since, according to the method, the optical path-length difference must be varied in one of the two arms. This necessitates a certain physical volume for the means for moving components in one of these arms. The use of the approach [7] requires means for varying the optical path length both in the reference arm and in the object arm. In many cases, for example for use in an endoscope, this can be achieved only with a comparatively high level of technical complexity, and with comparatively high costs.
White-light interferometry sensors based on the approaches described in [1]-[7] also in general do not allow measurements, for arrangements in which the volume must be greatly minimized, on objects with distances between the object and the sensor in the region of one or more meters, since during the measurement, the optical path length in the reference arm must at least once be made equal to the optical path length in the object arm. Even with folded arrangements, this generally leads to the sensor having a considerable physical volume.
The publication by T. Bajraszewski et al. “Improved spectral optical coherence tomography using optical frequency comb” [8] in Optics Express 17 Mar. 2008/Vol. 16, No. 6, pages 4163 to 4176 describes an OCT arrangement (OCT=Optical Coherent Tomography) having a frequency comb laser for the eye patient, in which the OCT arrangement includes a tunable Fabry-Perot interferometer in a frequency comb laser arrangement, and a spectrometer. The aim in this case is to improve the depth resolution of the OCT. A rapid single-shot measurement over an area is feasible only with a very high level of technical complexity, since an object is detected laterally in a serial form.
The document U.S. Pat. No. 7,391,520 B2 [9] discloses an OCT approach using a detector with a multiplicity of spectral channels, that is to say a spectrometer. The necessity to use a spectrometer in the optical measurement system in each case here means, however, that an object cannot be recorded over an area or as an image at one time, but in general can be recorded only point-by-point; the detection of an object over an area must therefore be carried out laterally and in a serial form. This is undoubtedly also acceptable for the applications described in [8] and [9]. Furthermore, however, these approaches do not allow application to the measurement of macroscopic objects, but are restricted for financial reasons to the measurement of comparatively small objects. In fact, it is not even possible to measure objects with a large depth extent and a long distance, using approaches such as these.
The publication by Choi, S.; Shioda, T.; Tanaka, Y.; Kurokawa, T.: Frequency-Comb-Based Interference Microscope with a Line-Type Image Sensor, Japanese Journal of Applied Physics Vol. 46, No. 10A, 2007, pages 6842-6847[10] describes an interference microscope with a frequency comb laser in which the frequency intervals are tuned. However, this approach cannot be used to completely measure an object with a comparatively great depth extent if the aim is to use a comparatively large numerical aperture for object imaging, in order to achieve high lateral resolution. Furthermore, if this approach is to be used for rapid measurement of an object with a comparatively large depth extent, it is absolutely essential to use either a high-speed camera or a short-pulse frequency comb laser source, or a rapid shadowing apparatus since, when the frequency intervals of the frequency comb laser are tuned through rapidly as is then required this also results in a high phase angle rate in the interference phenomenon to be scanned at the output of the interferometer. These means are either complex and costly or, in the end, lead to signals with a rather poor signal-to-noise ratio for detection of the interference phenomenon.
Known approaches using a second scanning two-beam interferometer associated with the object two-beam interferometer, as described in document GB 2355210 A by K. Ehrmann, produce interference signals with a reduced contrast, which can exacerbate signal evaluation. Furthermore, in this case, it is actually not possible to scale the measurement method for a large depth measurement range.
The document DD 240824 A3 by J. Schwider in 1972 described the use of a Fabry-Perot etalon in reflection, as an adjustment aid in a spectral white-light two-beam interferometer. In 1994, in the document DE 44 05 450 A1, J. Schwider likewise described the use of a very thin Fabry-Perot resonator in the beam path of a spectral white-light two-beam interferometer in order to still obtain interferograms that can be evaluated even for relatively long distances between an object and a reference surface in a Fizeau interferometer. This related to the visualization of interference. In this case, it is impossible to record objects using confocal filtering. In this case, the measurement method cannot be scaled for a large depth measurement range.
Furthermore, FIG. 7 in laid-open specification DE 3623265 A1 illustrates a Fabry-Perot interferometer for position measurement of a mirror in conjunction with a second interferometer for producing a spatially broadened interferogram. An arrangement such as this can be used to scan extended mirrors, but not small objects, since, in fact, sharp imaging of small objects using a multi-beam interferometer is in fact possible only to a very restricted extent.
The object of the present invention is to provide interferometry which can be adapted over a wide scale, with high measurement and scanning accuracy and with the measurement being highly robust.